2.71 Paricle System "Broken"

I am trying to use a simple burst of particles to make an explosion. I open a fresh session of Blender and add a particle system to the default cube, set start and end on frame 5 and lifetime to 1. This gives exactly the burst I need. But then, at or about frame 50 to 60, the particles begin emitting again! This time, they form on the surfaces of the cube and accumulate in ever greater numbers. As frame 250 is approached, the rate of re-emission slows and the life of the particles decreases until at 250, there are none there! I clear the cache each time. I vary all of the parameters. I push the start to frame 0. Nothing changes this behavior - but - every once in a while, it does in fact work as it should.

Any ideas about what is going on and how to fix it? Thanks, Dale.

Download a current build from https://builder.blender.org/download/

This seems to have done it! Thought I had the latest, guess not. Thanks. Dale